Weather

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fxus63 kdtx 062303 
afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
703 PM EDT Monday Oct 6 2008 




Aviation... 


High pressure will build into the eastern Great Lakes...pushing 
clouds across southwestern Michigan farther west. The high will 
bring VFR conditions to the taf sites through the taf cycle. 


&& 


Previous discussion...issued 357 PM EDT Monday Oct 6 2008 


Short term...tonight 


Cloud cover associated with sharp upper ridge across the western 
lakes and aided by entrance region of upper jet to our east is 
slowly dissipating as the jet departs and dry air filters in from 
large high pressure over Ontario. The upper ridge will slowly slide 
overhead tonight but cloud cover should continue to dissipate to 
just mainly scattered to broken cirrus. This will lead to mainly 
clear skies across most of the area with a bit more cirrus coverage 
across the western counties closer to ridge axis. 


Guidance temperatures vary quite a bit with met being the lowest and 
the mex highest. Will go in between as dont see surface dewpoints 
getting as low as NAM advertises. Surface gradient should be tight 
enough to keep winds up a little for a good part of the night keeping 
radiational cooling at Bay. Could still see patchy frost across The 
Thumb region where temperatures may reach the middle 30s in sheltered areas. 
Most areas should see upper 30s to low 40s. 


Long term... Tuesday through next Monday 


Nearly a typical day for early October is in store for Southeast 
Michigan on Tuesday. Persistent low level flow with an easterly 
component combined with thickening high clouds during the 
afternoon will confine high temperatures to the lower and middle 60s, 
right about normal for this time of year. These numbers are 
supported by model soundings that indicate a sharp low level 
inversion around 900 mb with the bulk of the warm advection 
occurring around the 850 mb layer. The easterly flow off of the 
surface high to our east and modest afternoon heating will limit 
the boundary layer depth as the surface front lingers in the Ohio 
Valley and awaits the plains system to bring it north Tuesday 
night and Wednesday. 


The pattern of showers in the plains today supports the 12z model 
solutions that show generous moisture transport taking place Tuesday 
night. Expect enough shower coverage by Wednesday morning to 
continue raising the chance of rain with this forecast cycle to 
around 70 percent for Wednesday. Dynamic forcing will be on the weak 
side in terms of short wave and upper jet support, but moisture 
transport will be deep and strong. This is judging by Theta-E 
advection in the 850 to 700 mb layer with close to 40 knots of wind 
resulting in 850 mb dewpoint around 10c by 12z Wednesday morning. 
This kind of moisture advection at night will be enough of a 
destabilizing influence to add a chance of thunder as well as the 
higher chance of rain. However, elevated instability as measured by 
850 mb lifted index is modest in the model data, just around 0c as 
the warm front moves through the region. Surface based instability 
ahead of the cold front during the afternoon is also quite weak due 
to limited heating also suggesting no more than a chance of 
thunderstorms with no severe weather. 


The Pacific nature of the front associated with this system will 
actually result in warmer temperatures on Thursday, more like what 
would be experienced on Wednesday if it were not for the rainfall. 
Guidance highs in the lower 70s look reasonable compared model 
1000-850 mb thickness and forecast soundings. 


Energetic upper level westerlies will begin to buckle Thursday night 
through Friday as strong height falls take place over The Rockies. 
This will lead to an expansive upper level ridge over the east half 
of the country and continued well above normal temperatures in the 
Great Lakes during this weekend. Now that there is good model 
agreement on the timing and location of the long wave amplification 
during Thursday, confidence is high on the forecast for the end of 
week as the large scale features continue to evolve. Predictability 
of the daily weather will be high with such large scale features in 
control. The next stage of the warming trend may be tempered 
initially on Friday by a lingering easterly low level flow off the 
broad surface high over eastern Canada, but will pick up momentum 
during Saturday and Sunday. Expect the upper ridge axis to drift to 
our east during this time and help deepen the southerly flow over 
our area. With dry conditions in place, high temperatures will run 
up well into the 70s compared to normal conditions in the lower half 
of the 60s for the second weekend in October. 


Differences in the model details do arise by Sunday into next Monday 
on the timing of the next front. The 12z GFS has trended toward the 
12z European model (ecmwf) but they still represent the two clusters of solutions on 
the pattern. The GFS has a greater degree of split flow by then and 
allows a northern stream wave to bring a front into the Great Lakes 
on Sunday. This solution, or one delayed until Monday, actually 
looks OK until we can get a better feel for the trends on the large 
scale flow over the North Pacific. 


Marine... 


Small craft advisories remain in effect through Tuesday morning for 
all marine areas except Lake St Clair. Easterly flow will persist 
but then weaken for a while Tuesday afternoon only to increase again 
Tuesday night. There could be another short period of higher waves 
Tuesday night before the winds turn toward the south by Wednesday 
morning. The winds will continue to turn toward the southwest 
Wednesday night after a round of showers move through the area with 
a frontal system. It's not really fair to call it a cold front since 
temperatures will actually be warmer with favorable wind conditions 
developing on Thursday. 


&& 


DTX watches/warnings/advisories... 
Michigan...none. 


Lake Huron... 
Small Craft Advisory...nearshore waters from Port Austin to Port 
Huron including Saginaw Bay...until 10 am Tuesday. 


Lake St Clair...none. 
Michigan waters of Lake Erie... 
Small Craft Advisory...until 10 am Tuesday. 


&& 


$$ 


Aviation.....Kurimski 
short term...drc 
long term....bt 
marine.......drc/bt 




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